The invention relates to a process for the production of internally-hardened glass tubes, devices for performing the process as well as uses of the tubes that are produced according to the process.
For numerous applications of glass tubes or of glass molded elements that are formed from glass tubes (semifinished products), high chemical resistance is necessary.
Hollow glass molded elements, which require an elevated chemical resistance of the inside surface, are, for example, those
for chemical plant production
which are used for flowmeters for chemically aggressive media
for analytical purposes (e.g., burette tubes, titration cylinders, etc.)
for reagent glasses for special purposes
for sheathings for measurement electrodes in aggressive media
for lighting purposes, e.g., halogen lamps
for discharge lamps
which are used as components for biotechnological reactors, and
which are used as containers for medical purposes (e.g., ampoules, vials, syringe bodies, cylinder ampoules, etc.).
It is known to produce glass tubes from silica glass (quartz glass, SiO2 glass) as semifinished products for shaping hollow glass molded elements, which have a very high chemical resistance. Such tubes are very production-intensive and costly, however, because of the high melting point of the SiO2 glass; they can also be produced only with limited optical quality and are not well suited for mass production. In addition, such tubes can be deformed only with very special devices, since, on the one hand, the deformation temperatures are very high, and, on the other hand, the temperature range in which deformations are possible is very small.
Semifinished product-glass tubes made of silica glass therefore cannot be produced with sufficient quality and economic efficiency for mass applications.
Predominantly low-melting glasses, e.g., borosilicate glasses or soda-lime glasses are therefore used for industrial glass tube products. The latter can be advantageously economically produced and deformed as tubes.
Processes that increase the chemical resistance of the inside surface of such glass tubes made of low-melting glass are already known.
Processes in which the glass surface is chemically leached out are known.
In this respect, a correspondingly aggressive gas, typically SO2 or HCl gas, is introduced into the still hot glass tube, which results in surface reactions and reduction of the alkali content in the surface.
Such dealkalizing processes are described in, e.g., H. A. Schaeffer et al.; Glastechn. Ber. 54 (1981) No. 8, pp. 247-256. The drawbacks of these processes are that mainly toxic gases are used, whereby the glass surface still can contain traces of these aggressive reaction gases according to the chemical treatment, and that the glass surface structure is damaged, which results in an increased surface area and in active centers of the surfaces. The use of such aggressive gases from environmental standpoints and worker-safety conditions is also disadvantageous. During deformation of such leached-out glass tubes, particles from the porous, damaged surface can dissolve. A washing process for removing the reaction products is also necessary before the leached-out glass tubes are used. This washing process makes necessary a subsequent drying and disposability of the reaction products, i.e., it increases the costs for the production of semifinished-product-glass tubes.
U.S. Pat. No. 3,314,772 describes another process for dealkalization of low-melting glass by fluorination using fluorine-containing compounds, e.g., aqueous HF solutions, which has the same typical main drawbacks as the other previously described processes for dealkalization.
To avoid the drawbacks of the dealkalization process, it is also known to provide tube-like glass containers that consist of low-melting glass, which are used especially as packaging for pharmaceutical materials, on their inside surface with a silicon oxide (SiO2) layer, which is comparable in its inertness to a quartz glass surface (M. Walther, xe2x80x9cPackaging of Sensitive Parenteral Drugs in Glass Containers with a Quartz-like Surfacexe2x80x9d from Pharmaceutical Technology Europe, May 1996, Vol. 8, No. 5, pages 22-27.
The coating of the inside surface of the formed glass molded element is carried out in this case by chemical deposition of oxidic coating material from its gas phase, especially using a vacuum-supported plasma-CVD process (PECVD=plasma-enhanced chemical vapor deposition), and especially using a pulsed plasma (PICVD=plasma-pulse-chemical-vapor deposition).
In the known case (DE 296 09 958 U1), the finish-formed containers, i.e., the glass molded elements themselves, are coated inside. As a result, each glass form container per se, matched to its form, must be subjected to an expensive coating process.
A feature of the invention is now to harden glass tubes that produce the semifinished product for the various hollow glass molded elements in a simple way on their inside surface.
In a tube-drawing process that is known in the art for the production of glass tubes, a coated drawing tool is used, for example in the Danner process as disclosed in U.S. Pat. No. 1,218,598; a coated Danner mandrel, or in the Vello process a coated Vello needle (see Heinz G. Pfaender, xe2x80x9cSchott Guide to Glass,xe2x80x9d Chapman and Hall, pp. 93-94 (1996 edition)) whereby the improvement is a coating that releases coating material to the glass surface upon contact with the inside surface of the tube that is produced.
By this xe2x80x9cdoping,xe2x80x9d the inside surface of the finished tube is hardened. It is passivated and has an elevated chemical resistance.
The release of the coating material should achieve a sufficient effect such that at least 1.5 xcexcg/(m2s) of the coating surface is released from the coating. The general release rate of the coating surface can be from about 1.5xe2x88x92about 15 xcexcg/(m2s), the preferred release rate of the coating surface can bexe2x89xa7about 5.0 xcexcg/(m2s), and the optimal release rate of the coating surface can be about 15 xcexcg/(m2s). This is ensured by the material that is used and its surface composition.
Suitable coating materials are inorganic materials, e.g., nitrides or preferably oxides, which are themselves sufficiently inert against water, acid or lye attack and in which sufficient diffusion and solution processes on the coating/glass interfaces occur at the temperatures and viscosities in this process step (about 700xc2x0 C.-1400xc2x0 C. and about 103.3 dPas-107.3 dPas). In the case of glasses with a transformation temperature of T9 less than 500xc2x0 C., only very small diffusion and solution amounts are produced because of the low process temperatures, and no significant degree of concentration of the coating material occurs on the inside tube surface. Since the time of contact between glass and coating is limited, coating materials with relatively high diffusion coefficients are effective.
Preferred materials have diffusion coefficients of at least 1xc3x9710xe2x88x9213 m2/s at operating temperature. The diffusion coefficients that are indicated for the materials relate to a temperature range of about 800xc2x0 C. to about 1200xc2x0 C., which is in the range of the operating temperatures (see above: temperatures in the process step): e.g., ZrO2: ≈3.8xc3x9710xe2x88x9211; with concentration proportions of about 0.1-5% by weight of RN, R3N4, RO2, RO, R2O3, doped SiO2: ≈7.7xc3x9710xe2x88x9211; Al2O3: ≈1xc3x9710xe2x88x9213. Especially suitable are ZrO2, Al2O3, SiO2, MgO and mixtures thereof, mullite, mixtures of the above-mentioned oxides with about 0.1-20% by weight of RO2, RO, R2O3, RN, R3N4, e.g., with Y2O3, or spinel (MgAl2O4) where R is Y, Ca, Mg, K, Si, Al, B, Ti, Mn, or Co. Coatings that consist of ZrO2 or with proportions of ZrO2, especially with proportions of at least 5% by weight of ZrO2, are especially preferred.
The coating can be applied with the commonly used processes to the drawing tool, which include, i.a., ceramic materials such as chamotte, sillimanite, zirconium oxide, zirconium silicate, spinel, cordierite, aluminum titanite, or metallic layers or metallic bases with a continuous temperature resistance of at least 1250xc2x0 C. These are, e.g., the LPPS (low-pressure plasma-sprayed) process and the APPS (atmospheric pressure plasma sprayed) process, whereby the last-mentioned process is especially suitable because of the formation of a porous layer.
The main element of each plasma coating unit is the plasma spray gun. Components of the plasma spray gun are tungsten electrodes and the copper nozzles that are arranged concentrically thereto. To produce plasma, an arc is ignited between cathode and anode (nozzle). In this case, the gas that is supplied concentrically to the cathode (often a mixture of argon and helium or hydrogen) is ionized and heated extremely strongly (average gas temperature about 10,000 K). The highly heated gas is greatly accelerated and forms the plasma flame outside of the plasma spray gun. Inside the nozzle, the spray powder is supplied with a feeder gas (frequently argon). Speeds of several 100 m/s are achieved. During impact on the drawing tool, the spray powder particles solidify and form interconnecting layers. Process parameters that are known to one skilled in the art, such as the angular position of the plasma nozzle and the grain size distribution of the plasma powder that is used, influence the resulting porosity of the layer. The open pore space of the coating zone should be more than about 10%, preferably at least about 15%.
Generally, layer thicknesses of between about 10 xcexcm and about 2000 xcexcm are produced. Layer thicknesses of about 10 xcexcm are already sufficient. The large surface area that is essential to achieve a sufficient effect can also be described by the specific surface area [m2/kg of coating agent] instead of by the porosity. It should be at least about 50 m2/kg, preferably about 75 m2/kg. The process according to the invention can be used in all tube-drawing processes that use a drawing tool, which is used to form the tube cavity or supports the latter. In this case, these are tube-drawing processes that are known in the art and proven, of which the most common are to be outlined briefly:
In the Danner process, a slightly tilted, slowly rotating tube, the Danner mandrel, on which a continuous strand of glass melt accumulates, is used as a drawing tool. At the lower end of the mandrel, the head of the mandrel, the glass is drawn off under the formation of the xe2x80x9cbulb,xe2x80x9d whereby a cavity is formed by supplying air through the hollow shaft of the mandrel. After deflection to the horizontal, the solidified tube passes through a gravity-roller conveyor to the drawing machine, behind which separation into tube sections is carried out by chopping.
In the Vello process, the glass melt that is already in tube form flows from the feeder, since it exits through a cylindrical nozzle. The melt flows through a mandrel, the Vello needle, the corresponding drawing tool of this process. Here, the glass is formed into a tube. Also here, the procedure is performed with blow air pressure. The tube first flows perpendicularly downward, it is then diverted horizontally and drawn off via a gravity-roller conveyor as in the Danner process, cooled and cut to length.
Also in the A-drawing process (down-draw process), the glass melt already flows in tube form from the feeder, since it exits through a cylindrical form. It flows over the drawing tool, a mandrel, here an A-drawing needle, where the glass is formed into a tube. In this process, the operation can be performed with air. The tube flows perpendicularly downward and is cut to length without deflection at temperatures of about 300xc2x0 C.
Both in the Vello process and in the A-drawing process, not only the needle element but also the holding shaft that is up to 2 m in length is coated. Both needle element and holding shaft are used combined here under the term needle or, in general, drawing tool.
Because of the larger glass contact surface area and thus the higher retention time, the Danner process is especially well suited for the process according to the invention. Depending on the size of the Danner mandrel, retention times of 0.5 to 4 hours are present at the coating material/glass interface, while the retention times in the Vello process and in the A-drawing process are only 30 to 50% of it.
The device according to the invention is distinguished in that a device for drawing glass tubes that is known in the art with device parts that are known in the art has a coated drawing tool, whose coating releases coating material on the glass surface upon contact with the inside surface of the glass tube that is produced.
A special device according to the invention with a tube-drawing unit according to Danner is distinguished in that an inclined, rotatable, axisymmetrical, coated mandrel is arranged in it. Thus, sufficient material from the coating is released on its inside glass surface when the glass tube is drawn, and the coating material that preferably contains ZrO2 and/or SiO2 and/or Al2O3 and/or MgO preferably has a diffusion coefficient of at least 1xc3x9710xe2x88x9213 m2/s. The preferred materials and their surface properties and layer thicknesses are mentioned in the description of the process.
Such a device for tube-drawing according to Danner has in addition a nozzle from which the glass runs from the feeder channel to the mandrel. It further has a furnace, for example a gas-heated muffle furnace, for setting a temperature gradient between the discharge from the nozzle and the end of the mandrel and a blowing device for imposing an overpressure or underpressure relative to the ambient pressure on the interior space of the glass tube that is to be drawn off.
In the Danner process, the temperature difference between the discharge from the nozzle and the end of the mandrel based on the type of glass and the tube that is to be manufactured is about 400 K. In this case, regardless of the type of glass, the viscosity range between the discharge of the nozzle and the end of the mandrel is 103.3 to 105.9 dPas based on the tube that is to be manufactured.
The build-up of the inside surface of the tube with the coating material can also be influenced to a small extent by:
the specific mandrel load in kg/m2xc3x97h
the introduction of energy in the gas-heated muffle furnace
the size of the muffle furnace
the heat conductivity of the material of the Danner mandrel
the material of the muffle furnace
the entire construction of the mandrel.
A special device according to the invention with a tube-drawing unit according to Vello or an A-drawing-tube-drawing unit is distinguished in that a coated tube-drawing needle is arranged perpendicular in it. Here, the same materials and layer thicknesses as described above are preferred as coating. Its diffusion coefficient is preferably at least 1xc3x9710xe2x88x9213 m2/s. It further has a preferably electrically heated muffle for controlling the deformation temperature and a blowing device for imposing an overpressure or underpressure on the interior space of the glass tube that is to be drawn off. The unit according to Vello has a deflecting device, with which the glass hose in the plastic state is deflected to the horizontal.
Instead of a coated drawing tool, an uncoated drawing tool can also be used in the process for the production of internally-hardened glass tubes, said tool which is itself sufficiently porous and which releases material, which is accumulated on the inside glass surface, during contact with the inside surface of the tube that is produced.
The properties and preferred implementations with respect to material, material release, diffusion coefficient correspond in a porous drawing tool to those of the porous coating of a drawing tool. A drawing tool that is made of ZrO2 or sillimanite is preferably used. The drawing tool is to have a porosity, i.e., an open pore space, of at least 10%. Of course, porous elements can be produced, for example, by sintering.
For this process with a porous drawing tool, the Danner process is especially suitable with use of a porous mandrel that is made of, for example, ZrO2 or sillimanite.
The release of the material is low both in the coated drawing tools and in the porous elements, so that it is unproblematic for the service life of the drawing tool.
Example: A drawing needle that is coated with 93% by weight of ZrO2 and 7% by weight of Y2O3 with a coating thickness of 400 xcexcm was run for 28 days at temperatures from 1240xc2x0 C. in an aluminosilicate glass with a glass throughput of about 5 tons/day. After this time, a reduction of the coating thickness by about 20 xcexcm (corresponds to about 5%) was used. From this, a service life of about 500 days can be projected. The service life of noble metal components is on average about 1 year.